Laser scribing is a process that is commonly used to cut linear grooves on the surfaces of substrates such as sapphire or silicon carbide wafers along streets formed on the substrates. Laser beams or sharply focused laser spots are generated for the process. Laser scribing may also singulate substrates by cutting entirely through the substrates when the substrates are relatively thin.
For laser scribing on substrates made of sapphire, the depths of the grooves formed are usually controlled to the order of microns, which requires precise cutting of the substrates. The focus of the laser beam also affects the depth of each cut. Therefore, it is essential to measure the height of a cutting surface of a substrate in order to regulate the depth of the cut by laser scribing. One method to measure the height of the surface of the substrate is by focusing on the surface of the substrate with an optical device. A good estimation of the height can be achieved by including more measurement points on the substrate. Generally, a laser processing apparatus incorporates a vision system which is used to capture images of the substrates for height measurement as well as for alignment before laser scribing may take place.
The estimation or measurement of the height of the surface of the substrate contributes to overhead time. Overhead time further includes the time taken for loading and unloading of substrates and for vision alignment. Typically, overhead time may be up to 60 seconds while the actual scribing time may be up to about 240 seconds to completely scribe a sapphire substrate with a diameter of 2 inches. During the overhead time, the laser head is kept idle and is therefore unproductive. If the number of vision and focusing points are increased, the overhead time will be even longer. This reduces productivity.
US Publication No. 2008/0290078 A1 entitled “Laser Processing Machine” discloses a laser processing machine that can simultaneously perform laser-cutting along two streets. This is done by splitting a laser beam into two separate laser beams using a prism. The two laser beams are of adjustable pitch so that they can operate simultaneously to cut a wafer at two separate positions. On the other hand, Japanese Publication No. 2008-110383 entitled “Laser Beam Machining Apparatus” provides a laser beam machining apparatus which can machine workpieces held on two chuck tables simultaneously by using a beam splitter to split a laser beam into two beams. The laser beam disperses along two separate routes for machining two workpieces held on the two chuck tables simultaneously.
While efficiency of machining is increased since two scribing laser beams are produced at the same time in the above prior art to scribe wafers at two separate positions, it is a challenge to manage two laser beams which have to be located at very close proximity to each other after splitting the source laser beam. It is technically difficult to control the quality and energy variation of two beams from a single source. This may tend to result in different scribing depths formed along different scribing lines. Moreover, the power of the laser head must be doubled to maintain the same laser energy level at the two positions, which increases the cost of the machine. Furthermore, during the overhead time, the laser head is still left idle and productivity is lost. As laser heads are expensive, it would be preferable to maximize usage of the same and to promote the efficient use of the laser heads.